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Abstract

This study reports the deposition of cuprous oxide [Cu2O] onto titanium dioxide [TiO2] nanowires [NWs] prepared on TiO2/glass templates. The average length and average diameter of these thermally oxidized
and evaporated TiO2 NWs are 0.1 to 0.4 μm and 30 to 100 nm, respectively. The deposited Cu2O fills gaps between the TiO2 NWs with good step coverage to form nanoshells surrounding the TiO2 cores. The p-Cu2O/n-TiO2 NW heterostructure exhibits a rectifying behavior with a sharp turn-on at approximately
0.9 V. Furthermore, the fabricated p-Cu2O-shell/n-TiO2-nanowire-core photodiodes exhibit reasonably large photocurrent-to-dark-current contrast
ratios and fast responses.

Introduction

UV photodetectors are important devices that have a range of commercial, research,
and military applications. They can be used for space communication, ozone layer monitoring,
and flame detection [1]. In recent years, high-performance GaN-based (including AlGaN and AlInN) [2-5], ZnO-based [6], and ZnSe-based [7] photodetectors have all been demonstrated. However, high-quality GaN-based UV photodetectors
could only be prepared on a sapphire substrate, which is much more expensive as compared
with a glass substrate. On the other hand, the photocurrent-to-dark-current contrast
ratio of ZnO-based UV photodetectors is still low. Titanium dioxide [TiO2] is a potentially useful wide direct-bandgap material (3.2 eV for anatase and 3.0
eV for rutile) for UV photodetectors, solar cells, and gas sensors due to its outstanding
physical, chemical, and optical properties [8-10]. TiO2 is a nontoxic naturally n-type semiconductor material which has a high-temperature
stability and low-production costs.

For two-dimensional [2D] films, TiO2 UV photodetectors such as metal-semiconductor-metal detectors and Schottky barrier
diodes have been demonstrated [11,12]. It is difficult to produce p- and n-type materials simultaneously, which is necessary
for certain device applications. Zhang et al. reported the formation of a 2D TiO2/Cu2O composite film for a photocatalyst application using the metal ion-implantation
method [13-15]. Cuprous oxide [Cu2O] is naturally a p-type direct-bandgap semiconductor with a cubic crystal structure
and a room-temperature bandgap energy of 2.17 eV [16], which makes it ideal for TiO2-based p-n heterojunctions. Cu2O can be deposited using methods such as thermal oxidation, anodic oxidation, sputtering,
solution growth, sol-gel, and electro-deposition [17-24]. Among these methods, sputtering is commonly used in the semiconductor industry.
By carefully controlling the growth parameters, high-quality 2D Cu2O films can be produced by direct-current [DC] sputtering [18].

Recently, one-dimensional oxide semiconducting materials have attracted a lot of attention
for potential application in optoelectronic devices due to their large surface-area-to-volume
ratio [25]. Wu et al. reported the growth of TiO2 nanowires [NWs] on glass substrates by the thermal oxidation-evaporation method [26,27]. They produced single-crystalline TiO2 NWs, whose size and density were controlled by adjusting the growth parameters. However,
no report on the fabrication of p-Cu2O-shell/n-TiO2-nanowire-core heterojunction UV photodetectors could be found in the literature,
to our knowledge. The present study reports the deposition of p-Cu2O film onto n-TiO2 NWs by DC sputtering and the fabrication of radial p-Cu2O-shell/n-TiO2-nanowire-core photodiodes. The physical, electrical, and optical properties of the
fabricated radial p-Cu2O-shell/n-TiO2-nanowire-core photodiodes are discussed.

Experimental section

Before the growth of TiO2 NWs, a Corning 1737 glass substrate (Corning Display Technologies Taiwan Co., Ltd.,
Taipei City, Taiwan) was wet-cleaned with acetone and deionized water. The glass substrate
was subsequently baked at 100°C for 10 min to evacuate moisture. A 400-nm-thick titanium
[Ti] film layer was then deposited onto the glass substrate by electron-beam evaporation.
Finally, the samples were annealed in a furnace at 700°C for 3 h to synthesize TiO2 NWs in argon [Ar] ambiance. The crystal quality of the as-grown NWs was then characterized
by an X-ray diffractometer [XRD] (MXP 18, MAC Science Co., Tokyo, Japan). The surface
morphology of the samples and the size distribution of the NWs were characterized
by a field-emission scanning electron microscope [FE-SEM] (JEOL JSM-7000F, JEOL Ltd.,
Tokyo, Japan).

To investigate the deposition of Cu2O, glass was used as the substrate. The target used to deposit Cu2O was a 4-N pure copper block mounted on the cathode. The distance between the target
and the sample was fixed at 60 mm. A rotating magnet fixed on the backside of the
cathode was used to enhance the plasma bombardment effect. During sputtering, the
Ar flow rate, deposition time, base pressure, and chamber pressure were kept at 15
sccm, 10 min, 2 × 10-6 Torr, and 6 mTorr, respectively, and the DC power, O2 flow rate, and substrate temperature were 200 W, 4 sccm, and 25°C, respectively. The
crystallography and structure of the deposited Cu2O and the Cu2O/TiO2 NWs were evaluated by XRD and FE-SEM.

Prior to the fabrication of p-Cu2O-shell/n-TiO2-nanowire-core photodiodes, a small piece of glass was used to cover the TiO2 NWs to prevent the deposition of Cu2O in these regions. A 200-nm-thick Cu2O layer was subsequently deposited onto the TiO2 NWs. A 500-nm-thick silver layer was then sputtered onto the Cu2O layer and TiO2 NWs to serve as the p-electrode and n-electrode with a shadow mask. Figure 1 schematically shows the structure of the fabricated p-Cu2O-shell/n-TiO2-nanowire-core photodiodes. A picoammeter (HP-4145B semiconductor parameter analyzer,
Agilent Technologies, Sta. Clara, CA, USA), connected via a GPIB controller to a computer,
was then used to measure the current-voltage [I-V] characteristics of the fabricated
diodes under darkness. The photo responses of the devices were also measured. During
photo-response measurements, a 4-W mercury vapor lamp emitting at 365 nm was used
as the excitation source.

Results and discussion

Figure 2a shows a cross-sectional FE-SEM image of the TiO2 NWs prepared on a Ti/glass template. It can be clearly seen that high-density TiO2 NWs of various lengths were grown on the Ti/glass template. As shown in Figure 2a, it can be seen that the average length, diameter, and density of these TiO2 NWs were 0.3 μm, 50 nm, and 60 wires/μm2, respectively. Figure 2b shows a cross-sectional FE-SEM image of the sample with Cu2O deposited on TiO2 NWs. As shown, the deposited Cu2O filled the gaps between the TiO2 NWs with good step coverage to form radial Cu2O/TiO2 NWs. It was also found that the deposited Cu2O formed at the sample surface after filling the gaps. In order to investigate the
coating performance of Cu2O, the deposited sample was scraped with tweezers into an alcohol solution, which
was then ultrasonically treated for 20 min to disperse the NWs. The solution was dropped
on carbon tape which was then placed on a hot plate to evacuate the alcohol. Figure
3a shows a SEM image of a single NW. Figures 3b and 3c show energy-dispersive X-ray [EDX] spectroscopic mapping images of Cu and Ti, respectively.
These figures correspond to the SEM image shown in Figure 3a. After the deposition of Cu2O, Cu and Ti atoms were distributed over the entire NW. These results suggest that
the sputtered Cu2O not only forms the head portion of the nanoclubs, but also forms nanoshells surrounding
the TiO2 cores in the nanowire portion of the nanoclubs. The formation of such p-Cu2O-shell/n-TiO2-nanowire-core heterostructure should be able to provide us with a large junction
area, which is important for the application of photodetectors.

Figure 3.FE-SEM and EDX images. (a) An FE-SEM image of a single radial nanowire. EDX spectroscopic mapping images of
(b) Cu and (c) Ti which were corresponded to (a).

Figure 4 shows the crystallographic characteristics obtained from XRD measurements. For the
pure TiO2 NWs used for adhesion, the peaks were attributed to the rutile-TiO2 (110) phase (JCPDS
Card No. 88-1175). For the p-Cu2O/n-TiO2 NWs, the peaks were attributed to the (110) and (111) phases of the Cu2O phase (JCPDS Card No. 78-2076). No Ti-related signal was found, indicating that
the Ti film changed into a TiO2 film after the annealing process.

Figure 5 shows the dark I-V characteristics measured from the fabricated radial Cu2O/TiO2 NWs. The rectifying behavior indicates that a p-n junction formed in the Cu2O/TiO2 NWs. The operation of the photodiode detector involves three steps (1) the generation
of electron-hole [e-h] pairs by the absorption of incident light, whose photon energy exceeds the bandgap
of the materials in the device; (2) the separation and transport of the e-h pairs by the internal electric field; and (3) the interaction of current with the
external circuit to generate an output signal. Hence, the I-V characteristics of a
photodiode in a dark environment are similar to those of a normal rectifying diode.
If the p-n junction does not form, the generated e-h pairs will exhibit an ohmic character in the I-V curve and change the resistance.
When a photodiode with a p-n junction is illuminated with optical radiation, the I-V
characteristics shift according to the photocurrent and reverse current. The measured
current in the photodiode, Im, is:

where Id is the dark current and Iph is the photocurrent. The presence of a reverse current indicates that the photo response
is due to the p-n junction, not the TiO2 NWs or the Cu2O. In the process of measurement under illumination, UV light passes through the TiO2 and illuminates the array of the radial p-Cu2O/n-TiO2 NWs; e-h pairs are produced in the radial NWs when the energy of the UV light is absorbed.
The e-h pairs are separated by the internal electric field, and a photocurrent is simultaneously
generated. Under forward bias, the turn-on occurred at approximately 0.9 V. With a
+5-V applied bias, the forward current of the device was 1.53 × 10-7 A, and with a -5-V applied bias, the reverse leakage current was 7.74 × 10-9 A.

Figure 6 shows the dynamic photo response measured from the fabricated p-Cu2O-shell/n-TiO2-nanowire-core photodiode. With a +10-V applied bias, the dark reverse leakage current
of the diode was only around 3.37 × 10-8 A. However, the reverse leakage current increased rapidly to 1.15 × 10-6 A upon UV illumination. When the UV lamp was turned off, the reverse leakage current
rapidly decreased to its original value. The reasonably large photocurrent-to-dark-current
contrast ratio and the fast responses suggest that the radial p-Cu2O-shell/n-TiO2-nanowire-core photodiodes proposed in this study are potentially useful for UV detector
applications.

Conclusions

The deposition of Cu2O onto well-aligned TiO2 NWs by DC sputtering was reported. With the proper sputtering parameters, the deposited
Cu2O filled the gaps between the TiO2 NWs with good step coverage to form radial p-Cu2O/n-TiO2 NWs that exhibited rectifying I-V characteristics. The fabricated radial p-Cu2O-shell/n-TiO2-nanowire-core photodiodes had a reasonably large photocurrent-to-dark-current contrast
ratio and fast responses.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

TYT carried out the nanowire experiments and data analysis and wrote the manuscript.
SJC and TJH participated in data analysis and revised and finalized the manuscript.
HTH designed the thin film and other experiments and data analysis. WYW participated
in the revision of the manuscript. CLH provided the concept of the growth process
of the nanowire. All the authors contributed to the preparation and revision of the
manuscript and approved its final version.

Acknowledgements

The authors would like to thank the National Science Council and Bureau of Energy,
Ministry of Economic Affairs of Taiwan, Republic of China for the financial support
under contract nos. 100-2221-E-006-040-MY2 and 100-D0204-6 and the LED Lighting Research
Center of NCKU for the assistance on device characterization.